Composting: how much rock powder to add it?

Many agronomists and producers ask me this question. I usually answer them with another: Are we talking about composting for temperate or tropical soils?

In Brazil, composting normally taught (and practiced) has always been that of countries with a cold or temperate climate. In this type of composting, when used to partially or totally replace mineral fertilizer, the dose per hectare is calculated in the same way as always. In other words, it is interesting to know how many kilograms of mineral nutrients we are providing to the soil, through that compound.

In this reasoning, it is common to hear that, by adding the rocks, we are “enriching” the compost. The amount of rocks to be added then depends on the respective guarantees of minerals, the percentage of these guarantees that would be available to the plants in the first harvest, the analysis of the soil and the need (extraction or export) of the crop.

To add crushed rocks to a tropical compound, the reasoning must not be the same. In tropical soils, if we want to produce well for many years, we have to be more concerned with flows than with stocks, both of minerals and, above all, of carbon.

In this sense, we know that, for a given soil and climate, it is the soil biology (biota + roots) that most impacts the speed and efficiency of these flows.
It is clear that each crop needs, to its degree, the presence of mineral nutrients in the soil. What I find quite questionable are basically two points:

1. The methods to calculate the efficiency of plant absorption of each agroecosystem, that is, of the total minerals applied to that soil, at that moment and in that way, how much will be effectively absorbed by the crop.
2. Methods for measuring the amounts of mineral nutrients available in the soil. Commonly soil samples are used, collected from 0 – 20 cm deep, submitted to universal extractors. The nutrients present in the deepest layers are not considered, nor those not evidenced by the extractors, but accessible according to the dynamic characteristics of the biology of that system.

Considering only these two points, we could already say that we are applying perhaps twice, sometimes only half, the mineral nutrients necessary for maximum productivity. If we apply half of what we need, we will reap less due to the lack of these elements.

If we apply twice as much, we will reap less because of the adverse side effects caused to the system, due to the overdose of synthetic or highly soluble mineral fertilizers.

For this, and for many other reasons, in tropical soils we need to understand how efficiently the element flows are working, within each particular system. It requires observation and experimentation, which is hard work, because generalizing is always easier.

Well then! So what would be the amount of rocks to be added, in a tropical compost? Answer: the most suitable compound for tropical soils is the one that best fulfills these two main objectives:

1. Provide the soil biota and plants with a top quality organic substrate, a mix of organic substances at different stages of decomposition, as a food source;
2. To be an abundant and powerful microbial multi-inoculant, to reinforce the population and the microbial diversity of the soil annually.

In order to achieve good tropical composting, it is essential that we interrupt, due to the lack of humidity, the course of composting, as soon as the maximum quality as a microbial substrate is reached. Does this mean that tropical composting is faster than temperate composting? Yea!

But… what about the rocks? Well, we know in practice that what determines, almost always, the maximum dose of applied compound/ha are the operational (machines/time) and economic (cost/ha) factors. So, to add the rocks, we have to decrease the organic amount of the applied compound. In this way, we would be “impoverishing” instead of enriching the compound.

For example: if the maximum viable dose is 6 /ha, in a given crop, when using a recipe of compost with 30% of rocks, we will actually be applying only 4.2t/ha of organic part (substrate + multi-inoculant) + humic substances).

Well, then, within this reasoning, is it better not to add rocks to the tropical compost?

Answer: not always! We often manage to reconcile the two things (organic part and rocks) in composting. This brings us at least two advantages: the first is operational, of course, due to the possibility of unifying two applications into one.

The second is the possibility of taking advantage of the temperatures and acids formed during the composting process, to promote and accelerate reactions between the two portions (organic and mineral), and thus make part of these minerals available and at the same time protected (linked to the organic) losses due to volatilization, leaching and adsorption.

There are several other issues involved, which do not fit here, but which also influence the decision on the quantities of each rock to be added. For example, agricultural gypsum, because of its industrial residues, after a certain dose (variable), begins to interfere negatively in the microbial activity of the compound.

Some rocks rich in magnesium, in high doses (above 20%), react with organic matter, generating losses of N by volatilization.
And … how have you been doing the recipes for tropical compounds then?

Answer: each case is different! If I consider all the compost recipes from our group of consultants (Libertas), since when we started until today, we will have additions of rocks that vary from 0 to 30% of the initial total dry weight. The average, in this case, would not represent much, because, as I said, the situations are particular.

To use in composting, we prefer to choose rocks that are more concentrated in those minerals that the case requires. The more concentrated the mineral, the less space we will occupy within the compound with that rock. Consequently, we will be able to apply a greater volume of the organic part.

Rocks with lower levels of nutrients can be extremely interesting to remineralize the soil and, in fact, there are several very good ones. However, in our view, they should be applied to haul, directly on the ground. The application of the tropical compost, in this case, must be done after the rock formation, preferably after the first rains.

If the producer does not have operational limitations, after the compost is ready, the rocks in the windrow can also be applied and homogenized with the composting machine. This way you can save an application, although replenishments will, of course, be more frequent.

One last observation: during the composting process, the organic part can lose up to 50% of the initial dry weight, in the form of CO₂. The longer the composting time and the number of rotations, the greater the weight loss. As the rocks do not evaporate, they end up with a higher relative concentration in the compost.

In other words: the greater the losses of the organic part, the more concentrated in minerals the compost will be. This is another tip for those who still insist on evaluating the quality of an organic compound, by the levels of NPK that it has …

Antonio N. S. Teixeira
Executive Director – IBA